This invention relates generally to semiconductor lasers and, more particularly, to arrays of semiconductor lasers fabricated as single structures. There are a number of applications of semiconductor lasers that require relatively high output powers, such as space communications, laser printing, and optical recording. In recent years, much of the development effort in semiconductor lasers has been directed to increasing the power output from lasers in continuous wave (cw) operation, both for single laser structures and for phase-locked arrays of multiple lasers.
By way of general background, a semiconductor laser is a multilayered structure composed of different types of semiconductor materials, chemically doped with impurities to give them either an excess of electrons (n type) or an excess of electron vacancies or holes (p type). The basic structure of the semiconductor laser is that of a diode, having an n type layer, a p type layer, and an undoped active layer sandwiched between them. When the diode is forward-biased in normal operation, electrons and holes recombine in the region of the active layer, and light is emitted. The layers on each side of the active layer have a lower index of refraction than the active layer, and function as cladding layers in a dielectric waveguide that confines the light in a direction perpendicular to the layers. Various techniques are used to confine the light in a lateral direction as well, and crystal facets are located at opposite ends of the structure, to provide for repeated reflections of the light back and forth in a longitudinal direction in the structure. If the diode current is above a threshold value, lasing takes place and light is emitted from one of the facets, in a direction generally perpendicular to the emitting facet.
Various approaches have been used to confine the light in a lateral sense within a semiconductor laser, i.e. perpendicular to the direction of the emitted light and within the plane of the active layer. If a narrow electrical contact is employed to supply current to the device, the lasing action will be limited to a correspondingly narrow region, in a process generally referred to as "gain guiding." At high powers, gain-guided devices have strong instabilities and produce highly astigmatic, double-peaked beams. For most high-power semiconductor laser applications there is also a requirement for a diffraction-limited beam, i.e. one whose spatial spread is limited only by the diffraction of light, to a value roughly proportional to the wavelength of the emitted light divided by the width of the emitting source. Because of the requirement for a diffraction-limited beam, most research in the area has been directed to index-guided lasers. In these, various geometries are employed to introduce dielectric waveguide structures for confining the laser light in a lateral sense, i.e. perpendicular to the direction of light emission and generally in the same plane as the active layer.
A useful introduction to these and other considerations in the design of semiconductor lasers can be found in a paper by Dan Botez, entitled "Laser diodes are power-packed," IEEE Spectrum, June, 1985, pp. 43-53.
Early attempts to combine semiconductor lasers into one structure used gain-guiding to confine the light laterally, but these were sensitive to changes in drive current and other conditions. With the development of more reliable and precise crystal growth technologies, such as metal-organic vapor-phase epitaxial (MOVPE) growth, there was increased activity in the development of index-guided phase-locked arrays, some of which were capable of achieving hundreds of milliwatts of output power. More recently, the focus of attention in this area has been on developing structures that assure fundamental-array-mode operation and provide a single-lobed far-field light distribution pattern.
In general, an array of laser emitters can oscillate in one or more of multiple possible configurations, known as array modes. In the most desirable array mode, all of the emitters oscillate in phase. This is known as the 0.degree.-phase-shift array mode, and it produces a far-field pattern in which most of the energy is concentrated in a single lobe whose width is limited, ideally, only by the diffraction of light. The least desirable array mode is obtained when adjacent laser emitters are 180.degree. out of phase. This is the 180.degree.-phase-shift array mode, and it produces two relatively widely spaced lobes in the far-field distribution pattern. Multiple additional modes exist between these two extremes, depending on the phase alignment of the separate emitters. Most laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
In recent years, the most promising array structure for producing a diffraction-limited beam in the fundamental array mode involves the use of Y-junction arrays, the principle of which was apparently first described in a paper by K. L. Chen and S. Wang, entitled "Single-Mode Symmetric Coupled Laser Arrays," Electron. Lett., vol. 21, no. 8, pp. 347-49, April, 1985. Basically, the Y-junction array employs a configuration of waveguides that converge in pairs at Y junctions. The principle of operation is that the 180.degree.-phase-shift mode will couple only very poorly into a converging Y section, but the 0.degree.-phase-shift mode will couple easily. Thus the structure discriminates against the 180.degree.-phase-shift mode, and favors the production of a diffraction-limited beam.
Although the Y-junction structure favors the 0.degree.-phase-shift mode of operation, with a main far-field lobe centered at a zero position, use of the Y-junction structure does not necessarily ensure that sidelobes will be insignificant in intensity; and, due to power splitting at each Y-junction, the structure suffers from radiation losses. Accordingly, there is still need for improvement in the development of alternate approaches to the achievement of single-lobed, diffraction-limited output beams from laser arrays. The present invention addresses this need.